I've been running my new grill mounted STi-R together with the windshield V1 (3.863) over the last few days and I have to confess, after numerous encounters, that my STi-R consistently has the beating of my trusted V1 on Ka band. I've now had multiple 34.7 and 35.5 hits and every time, the STi-R has alerted before the V1 by a few seconds. I've also had a windshield mounted STi which was always close, but generally slightly inferior to V1 on Ka, but my STi-R has V1 beat, no doubt. I'm not sure whether it's the difference between detecting through the windshield glass vs. unimpeded, but regardless, I'm convinced of the the STi-R's Ka superiority based on my experiences to date.
I've had no obvious problem running the two detectors together and actually like the combo because of the benefit of the V1 directional arrows. It is worth noting that the STI-R also has remarkable rear sensitivity despite its far from optimal grill mount location (even more rear-sensitive in my opinion than a dashmounted X50 or STi, both of which I have used extensively before).
I know the price tag for the STi-R is excessive, but I have to confess that in my opinion, Beltronics have in the STi-R, made the most sensitive radar detector in existence. And this comes from a V1 addict.